Author Topic: Tek TLA614 power supply repair  (Read 4032 times)

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Offline TerraHertz

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Tek TLA614 power supply repair
« on: February 07, 2016, 10:24:19 am »
One of the thunderstorms recently apparently *also* took out the power supply of my TLA614. Which I leave in standby all the time due to the options data likely being in a Dallas sealed battery-backed RAM chip, in which the battery is sure to go flat soon.

Today I was rearranging my workbench layout. New arrangement of equipment. At the end, to check everything was correctly plugged in (new mains cord layout), I turned everything on. Except the TLA, which wouldn't. Quick check: both mains fuses blown. Replace fuses, try applying small V via variac. Lots of current draw with just a few volts. Great.

Taking the machine apart, it turns out the power supply comes out very easily. The whole thing is on one tray, after removing some connectors and a few screws it slides right out, even with the mains IEC and fuses attached.

Didn't take long to find a fault: one of the two FDA24N50 FETs in the input PFC stage is a dead short. But... getting that out was not so nice. One that really illustrates how far Tek have fallen.

Firstly, they contracted out the entire power supply design. The silkscreen says: "Designed by: Martek Power Inc."
  http://www.martekpower.com

  Sticker:
   Model: PS2251   Rev F
   P/N: 119-5806-02  Rev 2
   SN: 1195 - 2000
   IN: AC 100-240VAC 50/60/440Hz  6A typical
   OUT: 3.35VD/35A 5VD/22A 12VD/6A Vfan/2.5A
        5VSB/1A   380W maximum.

Well, there was zero chance of getting a schematic from Tektronix, and I suppose from Martek is also zero. Search on their site turns up nothing.

Also Martek don't care about serviceability at all. The PFC FETs cannot be removed without also removing their heatsinks from the PCB. And the heatsinks are soldered into the PCB and are impossible to remove from the PCB without massive heat capacity custom soldering head applied to the pins. Which are thick copper tabs, pressed into tight, non-round holes, then twisted over on the solder side, then soldered.

Fortunately I had my massive heat capacity custom lump-o-copper soldering tool.
Even with that the heatsinks had to be levered out, and pulled through-plating out in 4 pads out of 8 due to being such a tight fit even with solder molten.

It also turns out the through plating on _those_ holes is circuit-functional, so when putting this back I'll have to be careful to jumper and solder both sides - on these heavy copper heatsinks.

Now I have a question. There are plenty of FDA24N50 available on Aliexpress, and even some from au.element14.com (via stock in England.) But does anyone know of somewhere in Sydney I can buy 2 or 3 tomorrow (Monday)? I don't want to have the TLA sitting around in pieces for weeks while an Aliexpress order filters through the post.  Especially since I found it was dead, and opened it up, only minutes after opening up an old scope to fix some minor problems it has.

Fingers crossed that FET is the only fault. Because if it's much more, and there's no way I can get a schematic, I don't fancy my chances of fixing this.


« Last Edit: February 07, 2016, 10:29:39 am by TerraHertz »
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Offline EPTech

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Re: Tek TLA614 power supply repair
« Reply #1 on: February 07, 2016, 12:26:53 pm »
Hi there,

Have you measured the FET's after desoldering them? Are they still shorted out? I was just wondering because if some VDR on the mains side or a transil and diode over the SM transformer has shorted out, that may also cause a false short between the drain and source of the FET. I am sure you have already ruled that out, but I am saying just in case.


Kind greetings,

Pascal.
 

Offline TerraHertz

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Re: Tek TLA614 power supply repair
« Reply #2 on: February 07, 2016, 12:35:57 pm »
Hi there,

Have you measured the FET's after desoldering them? Are they still shorted out? I was just wondering because if some VDR on the mains side or a transil and diode over the SM transformer has shorted out, that may also cause a false short between the drain and source of the FET. I am sure you have already ruled that out, but I am saying just in case.

Yes. With them still in circuit even it was clear that one was a piece of metal (reads the same as the multimeter lead resistance), while the other I wasn't sure. After removing them both, one is still a total short, the other (and the flyback diode) seems good. But I'll set up a test circuit later and test it works. Desoldering those heatsinks is such a bastard, I don't want to have to do it twice.
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Offline nctnico

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Re: Tek TLA614 power supply repair
« Reply #3 on: February 07, 2016, 12:56:08 pm »
I'd used a small wrench to loosen the screws and undo them with the heatsinks in place. Fiddly but doable. Maybe it would have been easier to remove the large capacitors.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline SeanB

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Re: Tek TLA614 power supply repair
« Reply #4 on: February 07, 2016, 05:58:58 pm »
Trace the gate leads back to the controller and check it is still physically intact, and that the output stage still has its ESD diodes working as a rough sanity check. If this is dead your new FET's will either not work or will blow up again.  Note the PFC will be supplied it's start up supply from the main switcher, it will only have some high value resistors for mains voltage sensing.
 

Offline TerraHertz

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Re: Tek TLA614 power supply repair
« Reply #5 on: February 07, 2016, 07:26:25 pm »
I'd used a small wrench to loosen the screws and undo them with the heatsinks in place. Fiddly but doable. Maybe it would have been easier to remove the large capacitors.

Trace the gate leads back to the controller and check it is still physically intact, and that the output stage still has its ESD diodes working as a rough sanity check. If this is dead your new FET's will either not work or will blow up again.  Note the PFC will be supplied it's start up supply from the main switcher, it will only have some high value resistors for mains voltage sensing.

Heh. Nctnico's definition of 'fiddly' apparently includes literally impossible. The screws couldn't be got out of the stack even with the nuts undone. Not enough clearance.

I could see there were some small SMDs under the heatsink area. And since I'd like to have a hope of working out the circuit here and tracing back to the controller (which I can see), to check whatever I can, both heatsinks had to come off.
A pox on the modern 'no you can't have schematics' culture, and the lawyers it rode in on.


What my workshop needs, is some kind of mains power conditioning and spike clamping. But the supply is 3-phase, on a 63A breaker. And the various workshop power point circuits are balanced across all 3 phases. So I don't really know how I'd achieve useful lightning overvoltage clamping cheaply.
Which is a pity, since there's definitely going to be a lot more of this very electric weather. The Sun is well into the initial stages of quietening down into the coming new Maunder Minimum, solar wind is declining, and the resulting predicted rise in cosmic ray flux reaching the Earth is already showing up in measurements. Which all produces more dramatic weather events - oceans still warm, but atmosphere cooling fast, plus increasing high altitude cloud droplet nucleation.
  https://nextgrandminimum.wordpress.com/2016/01/30/cosmic-rays-in-earths-atmosphere-are-intensifying/
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Offline nctnico

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Re: Tek TLA614 power supply repair
« Reply #6 on: February 08, 2016, 05:36:11 pm »
AFAIK there are 3 phase overvoltage protectors. I wouldn't get those from Farnell et al but from a webshop which specialises in electrical components (switches, lamps, distributor panels, etc).
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline TerraHertz

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Re: Tek TLA614 power supply repair
« Reply #7 on: April 07, 2016, 02:08:16 pm »
My TLA 614 is repaired now and the Dallas NVRAM read and replaced.
Writeup here: http://everist.org/NobLog/20160331_lightning_luck.htm#tla614
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Offline SeanB

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Re: Tek TLA614 power supply repair
« Reply #8 on: April 07, 2016, 06:57:33 pm »
Lightning protection on 3 phase is pretty easy, you get the large arrestor blocks that fit in the main panel after the 63A breaker, and which are connected to the 3 phase lines, and to a really good ground along with the panel. This is the main incoming panel, and needs a little lead to get to the actual distribution panel, preferably with a few turns of cable to provide extra inductance in the line. The distribution panel has some lower rated units after the individual circuit breakers. The idea is to absorb in stages, and then let the final stage be a lot of VDR units in the actual socket outlets to clamp at point of use.

Must look to see if I still have the ones I got in an industrial board, which were connected to some 30mm bus bars at one end.
 

Offline TerraHertz

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Re: Tek TLA614 power supply repair
« Reply #9 on: April 08, 2016, 01:11:44 am »
Lightning protection on 3 phase is pretty easy, you get the large arrestor blocks that fit in the main panel after the 63A breaker, and which are connected to the 3 phase lines, and to a really good ground along with the panel. This is the main incoming panel, and needs a little lead to get to the actual distribution panel, preferably with a few turns of cable to provide extra inductance in the line. The distribution panel has some lower rated units after the individual circuit breakers. The idea is to absorb in stages, and then let the final stage be a lot of VDR units in the actual socket outlets to clamp at point of use.

Must look to see if I still have the ones I got in an industrial board, which were connected to some 30mm bus bars at one end.

I'd like so see pics of what they look like. I've seen inside quite a few distribution panels of all sizes (some of which I was disassembling), industrial and residential, and never saw surge arrestors.
My problems:
 Don't know where to get them. I guess I should ask at Turks.
 I'm virtually always broke, so need to find such arrestors 2nd hand/cheap/free.
 Would need three sets, since my setup is: grid 3phase power to house meter & breaker board, then 3phase 63A power via 43m of underground cable duct down to my main workshop's sub-board, plus a separate single phase 15A underground run to the old machine tools shed - which then has two sub-boards: a three phase one from the adjacent main workshop, plus the single phase one (bit of a relic of earlier times.)

Of course, just putting in one set at the main meter board would be a good start. Though, hmm... I don't think there's a primary circuit breaker, just a switch after the supply authority sealed fuses. Putting arrestors in after those fuses sounds like a good way to blow them regularly. Which could be costly. Or the cost of installing a main breaker.

Sigh. Better do it though.
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Offline SeanB

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Re: Tek TLA614 power supply repair
« Reply #10 on: April 09, 2016, 06:36:35 pm »
Will try to see tomorrow if they are in the big box of parts still Guy, the original 2 motor controllers ( for some 3 phase sewage pumps) were a tad large to keep around, even though the bus bars inside were truly massive, rated at around 2kA.
 


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